The present invention relates to a method of manufacturing a printed wiring board and, more particularly, to a method of manufacturing a printed wiring board having an oxidation-proof coating consisting of an imidazole-based pre-flux and formed on a circuit pattern consisting of copper or a copper alloy.
In general, a pre-flux for a printed wiring board is a flux applied in the process of manufacturing a printed wiring board, and the purpose of the pre-flux is to prevent oxidation on a circuit pattern surface consisting of copper or a copper alloy formed on the printed wiring board and protect this surface so as to prevent degradation of soldering properties until the printed wiring board is used. In order to achieve this purpose, the pre-flux requires the following properties. That is, adhesion properties to copper or a copper alloy constituting the circuit pattern must be excellent, and degradation and a change of properties are not caused by air, water, or gases, and the flux contains no halides or the like not to corrode copper or a copper alloy. In addition, the pre-flux forms a coating having good insulating properties, and the pre-flux has excellent compatibility with a post-flux to obtain excellent solder wetting properties during a soldering operation.
In an oxidation protection treatment performed by an imidazole-based pre-flux used as a conventional pre-flux having the above properties, as shown in FIG. 2A, a part mounting pad 2 consisting of copper or a copper alloy is formed at a predetermined position of an insulating substrate 1, and a solder resist 3 is applied to the insulating substrate 1.
In order to remove a copper oxide film formed on the circuit pattern surface consisting of copper and a copper alloy and including the part mounting pad 2, the resultant structure is dipped in an H.sub.2 O.sub.2 --H.sub.2 SO.sub.4 solution mixture (35% H.sub.2 O.sub.2 : 25 ml/l, concH.sub.2 SO.sub.4 :200 g/l) at 25.degree. C. for 30 seconds, washed with distilled water, and then dried at 130.degree. C. for 30 seconds.
As shown in FIG. 2B, at 40.degree. C. for 60 to 120 seconds, the resultant structure is dipped in an aqueous formic acid solution which has a pH of 1.0 to 4.0 and in which 0.2 to 2.0 wt % of imidazole and copper ions having a concentration of 200 to 1,000 ppm are dissolved, and the resultant structure is washed with distilled water and dried at 130.degree. C. for 30 seconds. In this manner, an imidazole-based pre-flux 5 is formed on the circuit pattern consisting of copper and a copper alloy and including the part mounting pad 2. This method is popularly known. As described in Printed Circuit Technique Handbook, pp. 818 to 825, Feb. 1987, the imidazole-based pre-flux 5 chemically reacts with copper to form a complex and to serve as an oxidation-proof coating.
In this conventional oxidation-proof treatment method using the imidazole-based pre-flux, an acid treatment is generally performed as a pre-treatment step to remove copper oxide formed on the circuit pattern surface consisting of copper or a copper alloy. In this case, although the copper oxide on the circuit pattern surface consisting of copper or a copper alloy is temporarily, completely removed by this acid treatment, nonuniform copper oxide is produced again by oxidation in water during washing with water in the sequential step or spontaneous oxidation in an drying step, and a copper impurity is present. Therefore, a uniform oxidation-proof coating consisting of an imidazole-based pre-flux can not be obtained.